A description will be given to an example of a conventional biosensor in terms of a glucose sensor.
A typical glucose sensor is obtained by forming an electrode system including at least a measurement electrode and a counter electrode on an insulating base plate by a method such as screen printing and then forming an enzyme reaction layer including a hydrophilic polymer, oxidoreductase and an electron mediator on the electrode system. As oxidoreductase used is glucose oxidase; as the electron mediator used is a small complex, an organic compound or the like, such as potassium ferricyanide, ferrocene derivative or quinone derivative. A buffer is added to the enzyme reaction layer as required.
When a sample solution containing a substrate is added dropwise onto the enzyme reaction layer in this biosensor, the enzyme reaction layer dissolves to cause a reaction of the enzyme with the substrate, which accompanies reduction of the electron mediator. After completion of the enzyme reaction, the substrate concentration in the sample solution can be determined from a value of oxidation current, which is obtained when the reduced electron mediator is electrochemically oxidized.
In this type of glucose sensor, a reductant of the electron mediator generated as a result of the enzyme reaction is oxidized at the electrode, to determine the glucose concentration from the oxidation current value.
Such a biosensor is theoretically capable of measuring diverse substances by using an enzyme whose substrate is an object to be measured. For example, when cholesterol oxidase or cholesterol dehydrogenase is used as oxidoreductase, it is possible to measure a cholesterol value in a serum to be used as a diagnostic indicator in various medical institutions.
Because the enzyme reaction of cholesterol esterase proceeds very slowly, with an appropriate surfactant added thereto, activity of cholesterol esterase can be improved to reduce the time required for the overall reaction.
However, the surfactant, as being included in the reaction system, has an adverse effect on hemocytes, making it impossible to measure whole blood itself, as done in the glucose sensor.
Thereat, a proposal has been made to provide a filter (hemocyte-filtering out portion) in the vicinity of an opening of a sample solution supply pathway for rapid supply of only plasma with  which is obtained by filtering out hemocytes therein filtered  in blood into a sensor. When the filter is inappropriately built in the sensor, however, the hemocytes captured in the filter are destroyed and hemoglobin dissolves out.
As hemocyte components get smaller to about the size of the hemoglobin, filtration of  filtering out the hemocyte components with the filter becomes difficult, whereby the hemoglobin flows into the sample solution supply pathway to cause a measurement error.
This is presumably caused by the fact that a difference in thickness between the filter before absorbing a sample solution and the filter expanded after absorbing the sample solution is not fitted to a gap between pressing parts for holding the filter from the top and bottom. When the gap between the pressing parts for holding the filter from the top and the bottom is too narrow for the thickness of the filter expanded, the filter is prevented from expanding. The pore size of the filter thus prevented from expanding cannot widen sufficiently, destroying the hemocytes as infiltrating thereinto.
As opposed to this, when the gap between the upper and lower pressing parts is previously set wide for the supposed thickness of the expanded filter, it is feared that the filter may be sliding during storage since each sample solution has a different hematocrit value (ratio of red cell volume), resulted from which degrees of expansion of the filter also differ, depending on sample solutions.
Moreover, when a filter is made thinner than a conventional one in order to reduce the amount of a sample solution, mere suction of the sample solution from the termination of a primary side portion of the filter, like a conventional method (Japanese Patent Application No. 2000-399056), reduces the amount of the sample solution that can be absorbed within a certain period of time. For this reason, plasma flows out of a secondary side portion of the filter at a slower rate, and the inside of a sensor, especially the inside of a sample solution supply pathway, is saturated with the plasma at a slower rate, resulting in longer measurement time.
As opposed to this, when a suction area is made wider for increasing the amount of the sample solution that can be absorbed within a certain period of time and then the sample solution is dropped thereonto from the upper part of the filter, the sample solution flows along the surface of the filter at a faster rate than it infiltrates into the filter. The sample solution having flown along the surface of the filter then flows into the sample solution supply pathway from the opening thereof connecting the sample solution supply pathway to the filter, which may lead to a measurement error.
It is an object of the present invention to provide a biosensor improved such that plasma with  which is obtained by filtering out hemocytes therein filtered  in blood reaches an electrode system with rapidity in order to obviate the disadvantages thus described.
It is an object of the present invention to provide a cholesterol sensor with high-accuracy and excellent response, whose object to be measured is whole blood.